1. Field of the Invention
The present invention generally relates to the field of semiconductor fabrication. More particularly, the invention relates to the fabrication of acoustic wave structures in semiconductor dies.
2. Background Art
Bulk acoustic wave (BAW) structures, which can be used in frequency control or filtering applications, can include a piezoelectric layer sandwiched between upper and lower electrodes and an underlying acoustic mirror structure. The acoustic mirror structure, which can include a number of alternating dielectric and metal layers, can be used to trap acoustic energy in the piezoelectric layer by reflecting acoustic energy. However, roughness at the interfaces between the layers of the acoustic mirror structure can reduce the effectiveness of the acoustic mirror structure by causing energy loss through acoustic wave scattering, which can be particularly detrimental in the gigahertz (GHz) frequency operating range. Thus, it is important that the acoustic mirror structure be fabricated such that each layer is as smooth as possible.
Conventional solutions for reducing roughness include polishing the mirror layers as they are deposited or polishing the top layer of the final acoustic mirror structure. However, polishing to a required level of smoothness can be difficult and can require expensive processing equipment. Also, since it is important in BAW structures to have repeatable acoustic mirror thicknesses so as to provide repeatable operating frequencies, the difficulty of repeatedly polishing materials at a fixed rate can cause the layer thickness to be changed in an uncontrolled and non-uniform manner. As a result, BAW structure performance and yield can be undesirably reduced.